The conventional function of the wet end of a paper machine is based on, for example, the so-called Fourdrinier principle, by which an endless rotating wire is covered with pulp which consists of a liquid and the fibres required for paper manufacture and which is herein also called a web. Water is removed from the web at several different steps by means of devices placed under the wire and including, for example, foils and various suction boxes.
The foils are stationary blades which are placed underneath the wire and which have a suction effect. By the suction effect, the wire is pressed against the foil, wherein due to the friction, the speed of the wire must be limited, or the suction effect of the foil must be reduced. Consequently, to have a sufficient dewatering capacity, several foils must be placed under the wire, and the friction causes wear of the wire. To increase the dewatering capacity, several foils may be placed in a box having an underpressure, wherein dewatering of the web takes place also between the foils. It is obvious that an increase in the suction effect will increase the friction and the wear of the foil. Liquid is also removed from the web by means of suction boxes in which the suction is stronger and circular perforations or openings are provided in the flat upper surface. Finally, liquid and moisture are removed from the web by means of a couch roll with a rotating mantle provided with perforations or openings and with a suction inside. The dewatering of the wire and the web is partly based on compression and partly on the effect of air sucked through the wire and the web. The couch roll must be placed in such a way that the couch roll does not cause harmful sprinkling of liquid downstream. Consequently, for dewatering, several different techniques are used in parallel, examples being mentioned in the document WO 81/02753.
An increase in the speed of the wire will result in an increase in the length of that part of the paper machine where said dewatering of the web takes place. On the other hand, an increase in the suction effect to increase the dewatering capacity will cause an increase in the friction and wear of the wire. Increasing the number of foils or suction boxes, in turn, will increase the length of the machine and the friction caused. Furthermore, the selection of the devices used for dewatering should not have a negative effect on the pulp, for example due to shaking, because it will affect the final quality of the paper web.
A suction box is presented in EP 0 639 667 A1. Said suction box comprises a roll rotating with the wire and having a mantle that is, for example, meshed. The roll is placed in the box under a negative pressure. Air flows through the wire and the mantle of the roll. A device for producing suction in the suction box is presented in WO 99/64667 A1.
The production of the suction requires a separate device to be placed somewhere and to be maintained, which causes costs.
WO 95/07387 A1 discloses a device used for controlling a felt and a web supported on it, applying a rotating roll whose perforated mantle is permeable to air. Blades are fixed to the inner surface of the roll to produce and control an air flow. The function of the device is to produce a suction to keep the felt attached to the mantle and to prevent the felt from shrinking. Some embodiments of said document show that the suction cannot be produced by means of the blades, but the device must be connected to a separate device for producing the actual suction and not just for making the function of the roll more efficient. It is also very difficult and expensive to manufacture the device and difficult to clean it, because the blades must be fixed to the inner surface of the mantle. Other rolls are also known which are provided with a perforated mantle and equipped with a suction by means of a separate device. U.S. Pat. No. 5,347,728 discloses a device in which the blades are also placed on the inner surface of the mantle, or fixed on the central shaft of the roll. The function of said blades is to prevent a turbulent or rotational air flow affecting the suction.